Control system and method for an optical amplifier

ABSTRACT

A method and system for controlling the gain of the amplification of an optical signal is provided that includes both electrical feedforward and feedback. In a feedforward portion of an optical amplifier, the method includes receiving an optical signal and measuring an input power. Based on the measured input power and a desired gain, a feedforward pump power is determined. The pump power is adjusted based on the determined pump power. In a feedback portion of an optical amplifier, an output power is measured and gain is determined based on the output power and the measured input power. The measured gain is compared to a desired gain and the pump power is adjusted based on that comparison.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation of U.S. application Ser. No.10/107,727 filed Mar. 26, 2002 and entitled “Control System and MethodFor An Optical Amplifier”.

TECHNICAL FIELD

[0002] The present invention relates generally to optical communicationnetworks and, more particularly, to a control system and method for anoptical amplifier.

BACKGROUND

[0003] Telecommunications systems, cable television systems, and datacommunications networks use optical networks to rapidly convey largeamounts of information between remote points. In an optical network,information is conveyed in the form of optical signals through opticalfibers. Optical fibers comprise thin strands of glass capable oftransmitting the signals over long distances with small loss.

[0004] Optical networks often employ wavelength division multiplexing(WDM) or dense wavelength division multiplexing (DWDM) to increasetransmission capacity. In WDM and DWDM networks, a number of opticalchannels are carried in each fiber at disparate wavelengths. Networkcapacity is based on the number of wavelengths, or channels, in eachfiber and the data rates of the channels.

[0005] To increase the signal strength over long distances, opticalcommunications systems typically include optical amplifiers at orbetween network nodes. The amplifiers typically include automatic gaincontrol (AGC) to maintain a desired amplification factor (gain) acrossthe amplifier. An optical amplifier may be used for each wavelength orchannel transported by a fiber; however, using one amplifier for allwavelengths reduces system costs.

SUMMARY

[0006] The present invention provides a control system and method for anoptical amplifier. In a particular embodiment, pure-electric feedforwardand feedback controls are provided for an optical amplifier to providesub-microsecond response time to fiber cuts and other fast channelchanging events.

[0007] In accordance with one embodiment of the present invention, asystem and method for controlling an optical amplifier includescontrolling the optical amplifier pump lasers with electricalfeedforward and feedback circuits. In the feedforward portion, based onthe measured total input power, a feedforward pump power is determined.The pump power is adjusted based on the determined pump power. In thefeedback portion, an output power is measured and gain is determinedbased on the output power and the measured input power. The measuredgain is compared to a desired gain and the pump power is adjusted basedon that comparison.

[0008] Technical advantages of the invention include providing animproved control system and method for an optical amplifier. In oneembodiment, an optical amplifier includes automatic gain control (AGC)with nominal feedforward and compensation feedback control that greatlyimproves response time down to sub-microsecond speeds for fast channeladding or dropping processes, such as, for example, a fiber cut. Inparticular, the nominal feedforward control monitors the total inputpower of signals into the amplifier, and provides a nominal pump currentto the pump laser of the amplifier based only on the total input power.Because the pump power is changed immediately after the change in anumber of channels of the input signal, no extra energy is stored in thegain medium of the amplifier and no excess population inversion isgenerated during the transition process; therefore, no gain excursionwill be generated. As a result, better protection is provided todownline network components by reducing the probability of a large powerspike or power drop. In addition, transmission errors caused by powerfluctuations are limited or minimized.

[0009] Another technical advantage of the present invention includesproviding a pure-electric controlled AGC for an optical amplifier. Inone embodiment, the electric controlled AGC includes the nominalfeedforward control and a compensation feedback control. Thepure-electric AGC provides fast response times without the need for newoptical components or extra pump power. As a result, costs of theamplifier and/or amplifier control are limited or minimized.

[0010] Still another technical advantage of the present inventionincludes providing nominal feedforward control for an optical amplifierwith aging factor compensation. In particular, pump laser aging willincrease the required pump current for an input power, which will affecta predefined nominal value of pump current used for feedforward control.Because the pump power is always monitored in the optical amplifier, theaging factor can be determined automatically by the built-in controlunit and an aging factor applied.

[0011] Other technical advantages of the present invention will bereadily apparent to one skilled in the art from the following figures,descriptions, and claims. Moreover, while specific advantages have beenenumerated above, various embodiments may include all, some, or none ofthe enumerated advantages.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] For a more complete understanding of the present invention andits advantages, reference is now made to the following description,taken in conjunction with the accompanying drawings, in which:

[0013]FIG. 1 is a block diagram illustrating an exemplary opticalcommunication system;

[0014]FIG. 2 is a block diagram illustrating details of the opticalamplifier of FIG. 1 in accordance with one embodiment of the presentinvention;

[0015] FIGS. 3A-B is a graph illustrating nominal feedforward andcompensation feedback control for the optical amplifier of FIG. 2 inaccordance with one embodiment of the present invention;

[0016]FIG. 4 is a graph illustrating the performance characteristics ofthe optical amplifier of FIG. 2 in accordance with one embodiment of thepresent invention;

[0017]FIG. 5 is a block diagram illustrating a multistage opticalamplifier in accordance with one embodiment of the present invention;

[0018]FIG. 6 is a flow diagram illustrating a method for controllingamplification of an optical signal in a gain medium in accordance withone embodiment of the present invention; and

[0019]FIG. 7 is a flow diagram illustrating a method for controllingamplification of an optical signal in a gain medium in accordance withone embodiment of the present invention.

DETAILED DESCRIPTION

[0020]FIG. 1 illustrates an optical communication system 10 inaccordance with one embodiment of the present invention. In thisembodiment, the optical communication system 10 is a wavelength divisionmultiplexed (WDM) system in which a number of optical channels arecarried over a common path at disparate wavelengths. It will beunderstood that the optical communication system 10 may comprise othersuitable single channel, multichannel, or bi-directional transmissionsystems. Optical communication system 10 may be a long-haul, metro ring,metro core, or other suitable network or combination of networks.

[0021] Referring to FIG. 1, the WDM system 10 includes a WDM node 12 ata source end point and a WDM node 14 at a destination end point coupledtogether by an optical link 16. The WDM node 12 transmits data in aplurality of optical signals, or channels, over the optical link 16 tothe remotely located WDM node 14. Spacing between the channels isselected to avoid or minimize cross talk between adjacent channels.

[0022] The WDM node 12 includes a plurality of optical transmitters 20and a WDM multiplexer 22. Each optical transmitter 20 generates anoptical information signal 24 on one of a set of distinct wavelengthsλ₁, λ₂ . . . λ_(n). The optical information signals 24 comprise opticalsignals with at least one characteristic modulated to encode audio,video, textual, real-time, non-real-time, or other suitable data. Theoptical information signals 24 are multiplexed into a single WDM signal26 by the WDM multiplexer 22 for transmission on the optical link 16. Itwill be understood that the optical information signals 24 may beotherwise suitably combined into the WDM signal 26. The WDM signal istransmitted in the synchronous optical network (SONET) or other suitableformat.

[0023] The WDM node 14 receives, separates, and decodes the opticalinformation signals 24 to recover the included data. In one embodiment,the WDM node 14 includes a WDM demultiplexer 30 and a plurality ofoptical receivers 32. The WDM demultiplexer 30 demultiplexes the opticalinformation signals 24 from the single WDM signal 26 and sends eachoptical information signal 24 to a corresponding optical receiver 32.Each optical receiver 32 optically or electrically recovers the encodeddata from the corresponding signal 24. As used herein, the term eachmeans every one of at least a subset of the identified items.

[0024] The optical link 16 comprises optical fiber or other suitablemedium in which optical signals may be transmitted with low loss.Interposed along the optical link 16 are one or more optical amplifiers40. The optical amplifiers 40 increase the strength, or boost, one ormore of the optical information signals 24, and thus the WDM signal 26,without the need for optical-to-electrical conversion. Signalregenerators may be provided as needed along the optical link 16.

[0025] In one embodiment, the optical amplifiers 40 comprise rare earthdoped fiber amplifiers, such as erbium doped fiber amplifiers (EDFAs),erbium doped waveguide amplifiers (EDWAs), and other suitable amplifiersoperable to amplify the WDM signal 26 at a point in the optical link 16.In other embodiments, for example, the optical amplifiers 40 maycomprise a neodymium doped fiber, a thulium doped fiber, a dopedwaveguide, or other suitable gain medium.

[0026]FIG. 2 illustrates details of the optical amplifier 40 inaccordance with one embodiment of the present invention. In thisembodiment, optical amplifier 40 includes a plurality of opticalcouplers 42, a plurality of photodetectors 44, an optical pump 46, and again medium 48. Optical amplifier 40 also includes a nominal feedforwardcontrol module 50, a compensation feedback control module 52, and aplurality of electrical links 56 connecting the components.

[0027] The optical coupler 42 and photodetector 44 on an ingress side ofthe gain medium 48 forms an input monitor 57 while the optical coupler42 and photodetector 44 on an egress side of gain medium 48 forms anoutput monitor 58. Input monitor 57 is operable to tap the ingressoptical fiber 16 to obtain an ingress optical signal, to measure a powerof the ingress optical signal, and to generate an input power signalbased on the power of the ingress optical signal. As described in moredetail below, the input power signal is provided to the nominalfeedforward control module 50 and the compensation feedback controlmodule 52. The output monitor 58 taps an egress optical fiber to obtainan egress optical signal, measures the power of the egress opticalsignal, and generates an output power signal based on the power of theegress optical signal. As described in more detail below, the outputpower signal is provided to the compensation feedback control module 52.Together, the nominal feedforward control 50 and the compensationfeedback control 52 provide a pure-electric automatic gain controlledoptical amplifier.

[0028] In the input and output monitors 57 and 58, optical couplers 42are each operable to split an incoming signal into discrete signals orotherwise passively generate discrete signals based on a single signal.The discrete signals may be identical in form and/or content or maysuitably differ. In one embodiment, each optical coupler 42 is a coupleroperable to tap the connected link 16 and provide an optical signal ofbetween 1-5% of the tapped signal from the link 16 to the correspondingphotodetector 44.

[0029] Photodetectors 44 are each operable to receive and measure theprovided optical signal and to generate a power signal based on theoptical signal. The power signal indicates or is indicative of the powerof the optical signal on the corresponding link 16.

[0030] Optical pump 46 is operable to receive pump control signals fromthe nominal feedforward control 50 and compensation feedback control 52and generate a pump energy signal based on the pump control signals. Inoptical pump 46, the control signals may be combined to form a singlecontrol signal for the optical pump 46. It will be understood that thecontrol signals from the nominal feedforward control 50 and compensationfeedback control 52 may be combined in one of the control modules orotherwise combined to provide a single control signal to optical pump46, or may otherwise act upon optical pump 46 to control the pump energysignal. Optical pump 46 may be a continuous wave laser or other suitableenergy source operable to provide electromagnetic energy capable ofamplifying an optical signal.

[0031] Gain medium 48 is operable to receive an optical signal and pumpenergy signal and amplify the optical signal with the pump energysignal. Gain medium 48 may comprise, for example, erbium (Er) dopedfiber, neodymium (Nd) doped fiber, thulium (Tm) doped fiber, an erbiumdoped waveguide, or other medium operable to suitably transfer pumpenergy to an optical transport signal comprising one or more trafficchannels. In the erbium doped fiber embodiment, the gain medium may, ina particular embodiment, have a length of between 5 and 100 meters orother suitable lengths.

[0032] Nominal feedforward control module 50 is operable to receive theinput power signal from the input monitor 57 and to generate a nominalor feedforward pump control signal based on the received input powersignal. The nominal pump control signal is provided by the feedforwardmodule 50 to optical pump 46.

[0033] In a particular embodiment, feedforward module 50 comprisesnominal pump power module 60 and aging factor module 62. In thisembodiment, nominal pump power module 60 is operable to receive theinput power signal from input photodetector 44 and determine a basenominal pump power based on the input power signal. Aging factor module62 is operable to adjust the base nominal pump power determined bynominal pump power module 60 based on an age or deterioration level of alaser associated with optical pump 46. The feedforward module generatesand transmits the nominal pump control signal to optical pump 46 basedon the nominal pump power determined by nominal pump power module 60 andaging factor module 62.

[0034] Compensation feedback control module 52 is operable to receiveoutput power and input power signals from monitors 57 and 58 andgenerate feedback pump control signal based on the received output powerand input power signals. In one embodiment, feedback module 52 comparesthe output and input power signals to determine a gain for the amplifier40 and generates a feedback pump control signal to increase, decrease,or maintain the current pump energy level to provide a specified gainfor the optical transport signal.

[0035] Feedforward module 50 and feedback module 52, as well as othersuitable components of optical amplifier 40, may comprise analogcircuitry, digital circuitry embedded on a chip, or otherwise suitablyconstructed. Feedback module 52 and feedforward module 50 may alsocomprise logic embedded in media. The logic comprises functionalinstructions for carrying out programmed tasks. The media comprisescomputer disks or other suitable computer-readable media, applicationspecific integrated circuits (ASIC), field programmable gate arrays(FPGA), digital signal processors (DSP), or other suitable specific orgeneral purpose processors, transmission media, or other suitable mediain which logic may be encoded and utilized.

[0036] In operation, an optical signal to be amplified is received at aninput or ingress side of optical amplifier 40 along optical link 16 andis split into two signals at the optical coupler 42 of the input monitor57. One part of the split signal passes through first optical coupler 42and travels along optical link 16 to a second optical coupler 42 where apump energy signal received from pump 46 is added to link 16. Thecombined signal travels along optical link 16 through gain medium 48where the signal is amplified. The amplified signal proceeds alongoptical link 16 to the optical coupler 42 of the output monitor 58 whereit is again split into two components. A first component travels alongegress optical link 16 out of optical amplifier 40 and continues throughthe network to the intended destination.

[0037] Returning to input monitor 57, the input photodetector 44receives the second part of the split signal provided by the inputoptical coupler 42, measures an input power of the signal, and generatesan input power signal based on that power. The input power signal istransmitted along electrical link 56 to feedback module 52 andfeedforward module 50. Similarly, in output monitor 58, the split signaltravels along an optical link 16 to output photodetector 44 wherein anoutput power of the signal is measured and an output power signal isgenerated based on that power. The output power signal is transmittedalong electrical link 56 to feedback module 52.

[0038] Feedforward module 50 receives the input power signal and, vianominal pump power module 60, determines nominal pump power, which is anapproximation of the pump power to achieve the specified or desired gainin the amplifier 40 based on the input power as communicated by inputmonitor 57. The determination may be based on a strictly linearrelationship between input power and pump power, a monotonicrelationship between input power and pump power, or other suitablerelationship, or may include an aging factor from aging factor module 62to compensate for pump degradation, or any other suitable algorithmbased on the network and amplifier configuration. A nominal pump controlsignal is generated by feedforward module 50 instructing pump 46 togenerate a pump energy signal based on the determined pump powerrequired. The pump control signal may comprise an electrical current atwhich the pump 46 is to operate to generate the determined pump power.It will be understood that the pump control signal may otherwisesuitably indicate to the pump the nominal pump power determined by thefeedforward module 50. The nominal pump control signal, as well as otherpower and/or control signals, may be any analog, digital, electrical, orother suitable types of signals.

[0039] Feedback module 52 receives the input power and output powersignals generated by the input and output monitors 57 and 58. Feedbackmodule 52 determines an actual gain of the amplifier by, for example,comparing the power signals from the monitors 57 and 58, resulting in anactual gain. Generally, gain is the ratio of output power to inputpower. The actual gain is compared to a desired gain and a feedback pumpcontrol signal is generated based on the comparison. For example, if theactual gain is lower than the desired gain a feedback pump controlsignal is generated and transmitted to pump 46 increasing the pump powerof pump 46 to increase the gain. Likewise, if the actual gain is higherthan the desired gain, a feedback pump control signal is generated andtransmitted to pump 46 directing pump 46 to decrease the pump power.Thus, the feedback pump control signal may indicate a positive ornegative adjustment to pump power rather than a specified pump power.The specified change in pump power may be in terms of a change inelectrical current at which the pump 46 operates.

[0040] Working together, feedback module 52 and feedforward module 50operate simultaneously, contemporaneously, and continually, perpetually,and/or intermittently to control pump 46 to provide a specifiedamplification (gain) of the amplifier 40 based on the changing inputpower and output power of the optical transport signal. In particular,feedforward module 50 instructs the pump 46 to generate a pump energysignal at approximately the appropriate level to achieve the desiredgain. The feedforward pump energy signal is adjusted in a time constantof one microsecond +/−5%, or between 0.8 to 1.2 microseconds orotherwise on the order of a microsecond so as to provide fast transientresponse. As used herein, “on the order of” means within a rangecentered around the target number. Feedback module 52 makes sloweradjustments (on the order of 100 microseconds) and instructs pump 46 toprovide a pump energy signal at the exact power level required toachieve the desired gain. In this way, factors such as channelallocation, wavelength-dependent response of the photodetectors, andother factors are accounted for. Thus, feedforward module 50 is operableto quickly adjust the pump power of pump 56 to a range that isrelatively near that of the desired pump power, while feedback module 50provides fine tuning to adjust the pump power to the exact levelrequired. The combination of the feedforward and feedback controlssuppress the transient gain excursion in the optical amplifier.

[0041] FIGS. 3A-B illustrate nominal feedforward and compensationfeedback control for optical amplifier 40 in accordance with oneembodiment of the present invention. In this embodiment, the nominalfeedforward value is determined based on an approximate linearrelationship between input optical power and pump power to achieve aspecified gain while compensation feedback fine tunes pump power tocount for actual channel allocation. In addition, the nominal pump poweris adjusted to compensate for laser aging.

[0042] For an amplifier with a particular gain, the pump power requiredto maintain a constant population inversion, and thereby minimize gaintilt, is a function of the input power and the channel allocations. Inthis function, a change in total input power dominates the change in therequired pump power, although a change in channel allocations alsocontributes to the change in the required pump power. In one embodiment,the nominal pump power is the average of the required for all of thepossible channel allocations at the same input power. The feedbackcompensation factor is the difference between the pump power required bya specific channel allocation and the nominal pump power, and is muchsmaller than the nominal pump power.

[0043] Referring to FIG. 3, a linear relationship 64 is utilized bynominal pump power module 60 to determine a pump power based on theinput power of an optical signal. This linear relationship 64 may bedescribed mathematically as, for example, P=f(I). The feedback module 52adjusts the pump power from the nominal value determined by the linearrelationship 64 to an actual value determined by the actual relationship66 which accounts for operating channel allocation. Thus, the feedbackvalue represents the difference, positive or negative, between thenominal value determined by linear relationship 64 and the actual valuedetermined by actual relationship 66. The input power may be inmilliwatts (mW) and pump power in milliwatts (mW). In an alternativeembodiment, the nominal pump power may be determined based on anon-linear relationship, a monotonic relationship, or other suitablealgorithm that determines an approximate pump power to provide aspecified gain based on input power of an optical transport signal. Inone embodiment, the algorithm may also compensate for pump degradationor aging.

[0044] In operation, in response to at least a change in input power,for example, caused by a change in a number of channels, the feedforwardmodule 50 adjusts the pump power based on the new value of the inputpower within one to three microseconds or less. For example, theadjustment may take less than one microsecond. During this time and/orthereafter, feedback module 52 fine tunes the adjustment to the pumppower required to achieve a desired gain. This fine tuning adjustment isrelatively slow compared to the adjustment of feedforward module 52. Forexample, the feedback function may require 100-300 microseconds tocomplete.

[0045] Referring to FIG. 3B, aging factor module 62 of feedforwardmodule 50 may utilize one or more linear relationships 68 between anoutput pump power and an input pump current to account for pump laseraging. For example, a pump current of I1 may be used by the pump 46 togenerate a pump power of P when the pump is first deployed in the system10. For a temperature stabilized pump laser, for example, the outputpower has a monotonic dependence on the pump current.

[0046] After operation for a period of time, in which pump 46 succumbsto aging, a pump current of I2 is needed by pump 46 to generate the samepower P. In this embodiment, the nominal pump value determined by thenominal pump power module 60 using linear relationship 64 may be inputinto the aging factor module 62 which may then generate the needed pumpcurrent based on the age of the pump 46. It will be understood thataging compensation may be otherwise determined and applied to thenominal pump power. For example, the age compensation may be calculatedbased on any suitable algorithm. For example, because amplifier 40 isalways monitoring the output power, the aging factor can beautomatically determined by the control circuit.

[0047]FIG. 4 illustrates the power fluctuation associated with a channeldrop event in system 10 in accordance with one embodiment of the presentinvention. In the illustrated example, the network 10 experiences a fastremoval of channels, from forty channels to one channel in 100 ns asmight occur during a fiber-cut. As shown by input power 65 a, the rapidremoval of channels effects a rapid decrease in input power. As shown byoutput power of the survival channels 65 b, the rapid decrease in pumppower by the feedforward module 50 in response to the fiber cut reducesthe power surge to less than 1 dB, well within the tolerances of mostnetwork components. In connection with other controls, feedback module52 provides fine tuning of the gain of the amplifier in a fewmilliseconds.

[0048]FIG. 5 illustrates an optical amplifier 70 in accordance withanother embodiment of the present invention. In this embodiment, opticalamplifier 70 is a multistage amplifier, consisting of two or more gainmedia and pumps. In addition, optical amplifier 70 also includes avariable optical attenuator 72 positioned between the first stage 74 andthe second stage 76 of the optical amplifier 70. Variable opticalattenuator 72 is operable to receive an optical signal and an attenuatorcontrol signal and attenuate the optical signal based on the attenuatorcontrol signal. The variable optical attenuator 72 is adjustable toattenuate at different levels to achieve a variable gain across theoptical amplifier 70.

[0049] Referring to FIG. 5, the first and second stages 74 and 76 eachinclude an input monitor 80, an optical pump 82 and a gain medium 85.The first and second stages 74 and 76 also each include a nominalfeedforward control module 86 coupled between the input monitor 80 andpump 82. The first and second stages 74 and 76 share a compensationfeedback control module 88 coupled between the input monitor 80 of thefirst stage 74 and an output monitor 90 of each stage 74 and 76. Thefeedback module 88 provides feedback to the pump 82 of each stage 74 and76. The monitors, pumps, gain mediums, and controllers may each beimplemented as previously described in connection with amplifier 40 ofFIG. 2. Optical fiber links 92 and electrical links 94 may also beimplemented as described in connection with optical amplifier 40. In anexemplary multi-stage embodiment, the total gain across the amplifier 70remains constant, while the gain across each stage 74 and 76 may vary.

[0050] In operation, a transport optical signal at an input side ofoptical amplifier 70 is tapped by input monitor 80 which generates aninput power signal based on the power of the optical signal. The inputpower signal is provided to a feedback module 88 and a feedforwardmodule 86. A first gain medium 85 is associated with a first feedforwardmodule 86. The first feedforward module 86 adjusts the pump powerassociated with the first pump 82 and gain medium 85. The relationbetween the feedforward signal and the input power may be limited by themaximum available power of the associated pump 82 in this stage 74. Inthis case, the feedforward signal saturates if input power is higherthan a certain value, and the gain of the first stage decreases.

[0051] The first-stage amplified signal passes through an output monitor90 that generates a first-stage output signal and transmits thefirst-stage output signal to compensation feedback control module 88along link 94. Compensation feedback control module 88 generates anattenuator control signal and transmits the attenuator control signal tovariable optical attenuator 72 along link 94. Variable opticalattenuator 72 receives the attenuator control signal and attenuates thefirst-stage amplified signal based on the attenuator control signal. Theattenuated signal proceeds to the input monitor 80 of the second stage76, which generates a second input power signal based on the power ofthe signal from variable optical attenuator 72 and the desiredsecond-stage gain. This desired gain may change because of the change inthe first-stage gain. The second input power signal is forwarded to asecond feedforward module 86, which, in turn, generates a control signalfor a pump 82 associated with a second gain medium 85.

[0052] The second-stage amplified signal passes from second gain medium85 through the output monitor 90, which generates an output power signalfor transmission to feedback module 88. Feedback module 88 compares theinput power signal received from the first input monitor 80 with theoutput power signal and the attenuation value of the variable attenuator72 to generate a feedback pump control signal that controls the pumps 82associated with each of the first and second stages 74 and 76 of theamplifier 70.

[0053]FIG. 6 illustrates a method for feedforward control ofamplification of an optical signal in accordance with one embodiment ofthe present invention. In this embodiment, the signal is amplified withsingle stage optical amplification, but it will be understood that asimilar process may be employed for a multi-stage optical amplifier.

[0054] Referring to FIG. 6, the process begins at step 150 wherein asignal is received by the optical amplifier and an input power of thesignal is measured. In one embodiment, the optical transport signal istapped by optical coupler 42 and the input power is measured byphotodetector 44 of input monitor 57. Next, at step 155, a feedforwardpump power is determined based on the input power. As described above,the feedforward pump power may be a simple linear comparison of inputpower to pump power, a monotonic comparison, or may be by a morecomplicated algorithm, as required by the particular system in which theoptical amplifier is a component. In one embodiment, this step isperformed by nominal pump power module 60 and aging factor module 62 offeedforward module 50.

[0055] Next, at step 160, the pump power of the pump laser is adjustedbased on the nominal pump power determined in step 155. The process iscontinuous or otherwise suitable repeats during operation of theamplifier, and therefore, returns to step 150, wherein an input power ismeasured.

[0056]FIG. 7 illustrates a method for feedback control of amplificationof an optical signal in accordance with one embodiment of the presentinvention. In this embodiment, the signal is amplified with single stageoptical amplification, but it will be understood that a similar processmay be employed for a multi-stage optical amplifier.

[0057] Referring to FIG. 7, the process begins at step 200 wherein asignal is received by the optical amplifier and an input power of thesignal is measured. In one embodiment, this step is performed by inputmonitor 57. Next, at step 205, the output power of the optical amplifieris measured. In one embodiment, this step is performed by output monitor58.

[0058] At step 210, the gain of the optical amplifier is determinedbased on the measured output power and the measured input power of steps205 and 200 respectively. In one embodiment, this step is performed byfeedback module 52. At step 215, a feedback pump power adjustment isdetermined based on the gain. The gain may be a constant desired gain ormay be an adjustable gain, as required by the particular network inwhich the optical amplifier is a component. In one embodiment, this stepis performed by a feedback module 52.

[0059] At step 220, the pump power of the pump laser is adjusted basedon the feedback pump power adjustment. For example, if the measured gainis lower than the desired gain, the pump power is adjusted upwards forincreased amplification. If the measured gain is greater than thedesired gain, the pump power is adjusted downward to provide for lessamplification. The process is continuous and, therefore, returns to step200, wherein an input power is measured.

[0060] Although the method of FIGS. 6 and 7 has been shown with specificsteps in a specific order, it will be understood that the steps may beperformed in a different order as appropriate, and other steps may beadded or omitted as appropriate in keeping with the spirit of thepresent invention. The processes of FIGS. 6 and 7 may be repeatedcontinuously or periodically, in parallel or otherwise. In addition, oneor more of the steps may be omitted during one or more cycles of themethod. For example, if the input power has not changed or not changedin a measurable or substantial way, steps 155 and 160 of FIG. 6(determining feedforward pump power and adjusting the pump power) may beomitted.

[0061] Although the present invention has been described with severalembodiments, various changes and modifications may be suggested to oneskilled in the art. It is intended that the present invention encompassany changes and modifications as fall within the scope of the appendedclaims.

What is claimed is:
 1. An optical amplifier, comprising: a gain medium;an input monitor, operable to measure a power of an ingress opticalsignal and generate an input power signal based on the power; an outputmonitor, operable to measure a power of an egress optical signal andgenerate an output power signal based on the power; an automatic gaincontroller including a feedforward module and a feedback module; thefeedforward module operable to receive the input power signal generatedby the input monitor and to generate a first control signal based on theinput power signal; the feedback module operable to receive the inputand output power signals generated by the input and output monitors andto generate a second control signal based on the input and output powersignals; and the automatic gain controller operable to control pumpenergy provided to the gain medium based on the first and second controlsignals.
 2. The optical amplifier of claim 1, wherein the optical pumpis a continuous wave laser.
 3. The optical amplifier of claim 1, whereinthe gain medium is erbium doped fiber.
 4. The optical amplifier of claim1, wherein the input and output power signals generated by the inputmonitor and output monitor are electrical signals.
 5. The opticalamplifier of claim 1, wherein the first and second control signalsgenerated by the feedforward module and feedback module are electricalsignals.
 6. The optical amplifier of claim 1, wherein the first controlsignal is based on the input power, an aging factor, and the desiredgain of the amplifier.
 7. The optical amplifier of claim 1, wherein thecontrol signal generated by the feedforward module is linearlyproportional to the input power signal.
 8. The optical amplifier ofclaim 1, wherein the control signal generated by the feedforward moduleis monotonic to the input power signal.
 9. An optical amplifier,comprising: a pump laser operable to generate pump energy; a gain mediumcoupled to the pump laser and operable to amplify an optical signal withthe pump energy to generate an amplified optical signal; a controllercoupled to the pump laser and the gain medium, the controller operableto control the pump laser based on feedforward monitoring of the opticalsignal and feedback monitoring of the amplified optical signal.
 10. Theoptical amplifier of claim 9, the controller further operable to controlthe pump laser based on an aging factor of the pump laser.
 11. Theoptical amplifier of claim 9, the controller further operable to controlthe pump laser based on a desired gain of the amplifier.
 12. The methodof claim 9, wherein the optical pump comprises a continuous wave laser.13. The method of claim 9, wherein feedforward monitoring is based on alinear relationship between the input power and a pump current of anoptical pump.
 14. The method of claim 9, wherein feedforward monitoringis based on a monotonic relationship between the input power and a pumpcurrent of an optical pump.
 15. The method of claim 9, whereinfeedforward monitoring is based on a linear relationship between theinput power and a pump current of a set of optical pumps.
 16. The methodof claim 9, wherein feedforward monitoring is based on a monotonicrelationship between the input power and a pump current of a set ofoptical pumps.